Modified total intensity ratio methods for measuring cell gap of twisted nematic liquid crystal cells

This paper modifies the total intensity ratio method (TIRM) used to measure the cell gap of twisted nematic liquid crystal (TNLC) cells. Compared to the conventional TIRM in which a mechanical mechanism is used to physically rotate the polarizer, the modified TIRM methods presented in this study measure the total intensity ratio using a polarization rotation modulator. Two modified measurement methods are introduced. In the first, the saw-tooth signal applied to the electro-optic (EO) modulator is used as a reference signal in order to determine the polarization state of the measured signal. In the second method, a beam splitter and an additional quarter wave plate are added to the optical configuration. The quarter wave plate is adjusted such that a phase-matching condition is obtained between the reference and measured signals. The experimental results confirm that the modified TIRM approaches yield comparable accuracy to the conventional TIRM. Furthermore, in the proposed approaches, sufficient intensity signals to determine the cell gap can be obtained in just 2s.